Pick up the super shotgun in front of you and take the dark passage to the next room. A large horde of imps and shotgun guys is ahead, so destroy them and head north through the next hall. Keep going until you see the yellow skull key behind some bars. You cannot reach it yet, of course, so take the nearby steps down to a lower area. Look out for a teleporting monster ambush, then press the lion switch, being wary of a few more enemies coming in. Now head southeastward and kill the enemies around this area. You will want to head south towards the mancubus, as it is the only place you can go at the moment (the other paths are a dead end and the blue door), and then east and up to the next door. The large outdoor area that follows has cacodemons and three paths. The north and south ones require keys, so go through the east path and into the teleporter. When you emerge, you will see a spider mastermind on top of the red skull key. Kill him and grab the key.

To leave this area, you need to press the two lion switches on the east and west sides, looking out for many demons that attack you as you press them. This opens up the north side, with a berserk pack and another switch, which opens up the teleporter to the south. Taking it takes you back to the previous open area. With the red key, take the red door to the north. Kill the monsters as you make your way to a room with another berserk pack and a switch. This one opens a passage in the previous hallway, which takes you back to the yellow key, which is now accessible. As always, prepare for a few teleporting monsters, and then return to large open area. With the yellow key, take the yellow door to the south. There will probably be a lot of shotgun guys and some mancubi running about, so dispose of them and continue until you reach a switch. Although it does not lower the nearby barricade, it does open up a wall leading to a different passageway. Keep going until you reach a soul sphere. Grabbing it triggers the barricade from before to lower and many enemies will teleport around the area. The room north of the soul sphere contains the blue key, so do not forget it. With all of the keys now in your inventory, head back to the blue key door that you remembered from before. Kill the chaingunners and arch-vile and hop in the teleporter. You will be back at the spiderdemon/demon area, and this time, the northern switch has been lowered. Follow the path behind it to find the exit.

From the start, go through the tunnel to the left. In the room at the end of the tunnel, open the tan colored pentagram on the wall to reveal a blue armor.

In the room with the blue-key door, turn around from the door and look to the south. From the pillar you see there, walk strictly to the west. At the end of the hallway, push the northern part of the western wall to reveal a room with a chainsaw and a backpack.

In the hallway where you first see the blue skull key, open the differently textured wall to the left of the exit door to reveal a soul sphere.

After picking up the blue skull key, go to the hallway north of it. Open the differently textured wall to the right to reveal a room with a rocket launcher.

Depending on the skill level, between 11 and 17 monsters cannot be killed without cheating:

Four holding areas feeding teleport traps will not open up, as there are no linedefs affecting sector tag 10. This prevents 100% kills in normal playing conditions, as four (easy skills) or eight (medium and hard skills) shotgun guys will stay trapped in their holding boxes.

Similarly, there are no sectors tagged 42, therefore another teleport monster box is rendered inoperative, as the four shotgunners (easy skill) or two shotgunners and four chaingunners (medium and hard skills) remain stuck, unable to reach a destination.

A third teleport box is broken, again by the lack of sectors tagged 42. It contains a demon, a chaingunner, and (on medium and hard skills) a Hell knight.